What helped our stallion was a cough syrup (natural) that I can't remember the name of but it had a strong menthol smell and it really opened up the passages.  He liked it so much he would lick it out of a bucket; I didn't have to force it in him.  But the menthol/eucalpytus will help open what breathing passages have not been damaged.  I don't know if you can find it on the Internet; you might have to go sniff the cough medicines, and find the one with the heavy menthol aroma.  (It will open your nose, too.  I leaned my head over the bucket and it opened my nose and lungs).

Forgot to add in my previous post that I have asthma; and regular exercise keeps my lungs in much better shape; and I have less asthma when my immunity is high.  Immunity plays an important part, which is why I'm trying to boost my mare's immunity.  My stallion isn't haven't problems anymore, since he is in an open barn.